Osmotic conditions could promote scFv antibody production in the Escherichia coli HB2151

Introduction

Single chain variable fragment (scFv) antibodies are the reduced forms of the whole antibodies including their original antigen detection performance along with other specificities such as high penetration activity in the tumor tissues, low side effects and short clearance time.^1-3 Regarding high dose consumptions of the antibodies for therapeutic tasks^4,5 and large amount demands for these recombinant proteins due to vast investigations for diagnostic purposes, optimization of the production processes makes sense. Antibody engineering is one of the strategies has been applied for the enhancement of the scFv production via promoting secretion system efficiency.^1,6-8 Selecting hosts other than Escherichia coli (commonly used platform) had been another strategy for high yield scFv production.^9-12 Metabolic considerations could be an effective approach to influence the induction process toward higher recombinant protein production.¹³

Inducer type and its utilized concentration are of the determinant factors could alter the induction process. Isopropyl β-D-1-thiogalactopyranoside (IPTG) is the highly applied inducer in the research area but other inducers such as arabinose and xylose which are cheaper pentose sugars could be applied with the low concentrations and lower toxic effects.^7,11

Apart from molecular aspects of the induction process, it could be affected by environmental conditions controlled by physical and chemical factors. Environmental parameters such as media composition, induction temperature, and agitation speed could control, inhibit or amplify the induction process.^14,15 It has been shown that sugars such as sucrose and sorbitol generating osmotic condition show the capability of increasing the protein solubility^16,17 and could be added to the induction media.

Regarding the economical aspect of the recombinant protein downstream processing, the destination of the expressed proteins in the cell become important because it determines the extraction and purification process complexity. Location of the accumulation of the target protein not only affects its folding and aggregation properties but the number of the steps required for extraction and purification processes. Because of the importance of the protein product secretion, a number of researches have been conducted to enhance the E. coli secretion efficiency.^18,19 Optimization of the environmental parameters could influence the target protein localization that is of great importance for the industrial scale recombinant protein production.^20,21

The aim of this study was to detect and optimize the effective factors of the lab scale production of scFv antibody by the induction start in the stationary phase of the E. coli HB 2151. In the current investigation, the statistical experimental design was applied to study the impact of several physical and chemical factors on the periplasmic and culture media scFv. The concentrations of sucrose and sorbitol, as effective factors on the periplasmic scFv production, was selected to impose osmotic conditions in the culture media, then the osmotic region for the optimum induction was obtained.

Material and Methods

Results

Periplasmic extraction optimization

Extraction of the expressed target protein is one of the major steps in the production process. In this work TSE (Tris-sucrose-EDTA) buffer was selected as an extraction solution to capture the periplasmic secreted scFv. The concentration of Tris as a key component of the extraction buffer and contact time between buffer and membrane are effective parameters that could increase the outer membrane permeability subsequently the amount of the released periplasmic scFv.²⁵ To select the optimum level of these factors, concentration of Tris in three levels 50, 100 and 200 mM and contact time in two levels 0.5 and 1 hour was selected. All efficient combinations were tested for periplasmic extraction of the overnight induced cultures (Fig. 1). Two experiments including 50 mM Tris (0.5 hour) and 200 mM Tris (1 hour) were omitted because of the very low scFv extraction and cytoplasmic protein pollution during the periplasmic protein extraction respectively. It is clear that using 200 mM Tris extraction buffer with 0.5 hour contact time causes higher scFv capture in a short time (high productivity).

Fig 1. Periplasmic extraction buffer with different Tris concentrations and contact times were applied on the same amount of the biomass and the extracted scFv production yields (Y_P/X) were compared.

Screening of the effective factors

To find the effective parameters on the scFv production, 10 physical and chemical parameters were considered and 12-run Plackett-Burman design (Table 1) as a screening method was used to evaluate their impact on the scFv production in the periplasmic space and in the mineral culture media.

Concentrations of scFv in the periplasmic and culture media (mg/L of the culture media) were selected as two responses (Table 1) and the main effect graphs were obtained (Fig. 2). There was a relatively same behavior for periplasmic and culture media scFv concentrations in terms of the selected parameters.

Fig. 2. Main effects plot for extracted periplasmic (A) and culture media (B) scFv concentrations (mg/L of media).

The results of the experiments considering the periplasmic scFv production (Fig. 2A) show that the concentrations of (NH₄)₂SO₄ (N source), NaCl could not be considerably effective in the determined region. Arabinose and MgSO₄ concentrations show higher performance in their lower levels.

On the other hand temperature, agitation and filling volume fraction parameters showed good results at their lower levels so these parameters were fixed on 30˚C, 200 rpm and 1/10 respectively.

Induction duration effect shows a slightly positive slope, then 24 hours induction was chosen for the next experiments despite the inverse effect on the productivity.

Sorbitol and sucrose as well-known sugars imposing an osmotic condition in the induction culture media,²⁶ showed a clearly positive effect on the periplasmic scFv production then the extended design of experiments were introduced to find the extent of osmotic condition could be applied in the induction media to promote the scFv production.

Data obtained from the scFv and DNA concentrations in the culture media (Fig. 3) show that both have relatively the same behavior so wherever scFv concentration rises subsequently DNA concentration is increased which demonstrates that the produced soluble scFv in the media is resulted by the cell lysis¹³ hence that is not a favorable product because of cytoplasmic and no oxidized scFv protein contaminations. Therefore, the responses related to the concentration of scFv in the culture media were neglected.

Fig. 3. Concentration of scFv and DNA detected in the culture media for 12 runs of placket-burman design.

CCD to find the optimum osmotic condition to produce periplasmic scFv

As described in the previous section, to improve the periplasmic scFv production, osmotic mineral media were used. Sucrose and sorbitol were utilized as key components to be added to the induction media and their concentrations were selected as independent variables. A 2-level full factorial experimental design with two center points was constructed then replicated in different blocks (Table 2) and the concentration of the extracted periplasmic scFv (mg/L of media) was set as the main response.

Table 2. Full factorial design with center points
Run	Blocks	X 1	X 2	Y
1	1	0.4	0.4	2.8
2	1	0.6	0.4	1.7
3	1	0.4	0.6	1.8
4	1	0.6	0.6	1.2
5	1	0.5	0.5	2.5
6	1	0.5	0.5	2.4
7	2	0.4	0.4	2.5
8	2	0.6	0.4	1.8
9	2	0.4	0.6	1.9
10	2	0.6	0.6	1.5
11	2	0.5	0.5	2.5
12	2	0.5	0.5	2.1
13*	3	0.331821	0.500000	2.5
14*	3	0.668179	0.500000	1.6
15*	3	0.500000	0.331821	2.5
16*	3	0.500000	0.668179	1.5
Axial points (*) were added to modify the design (b). All factors are in uncoded units.

The effect of main factors X₁ (sucrose molarity) and X₂ (sorbitol molarity), their interaction and curvature effects were checked (Table 3). It is clear that unlike the interaction effect, two main effects (i.e., sucrose and sorbitol molarity) and curvature effect are significant with 95% confident level (P < 0.05 ) in the study range that are shown graphically in the effect plot (Fig. 4). Also, block effects are not significant, which illustrates the repeatability of the replicated tests in different blocks. Therefore, this design was extended to a CCD by adding 4 axial points to the full factorial design with α = 1.41 (Table 2) to cover the extended space and fit a full quadratic model to the response.

Table 3. Main effects in the full factorial design
Term	P
X1 (Sucrose molarity)	0.002
X2 (Sorbitol molarity)	0.004
X1.X2	0.098
Center point	0.006
Block	0.881
The center point effect is significant (P <0.05) then there is a curvature effect.

Fig. 4. Normal plot of the standardized effects for full factorial experimental design. Sucrose (A) and sorbitol (B) are significant factors.

Using the new extended design data, a reduced quadratic model without interaction term (non-significant) was developed as follows:

where, the coefficients are in uncoded units.

The adequacy study of the model was performed first by residual analysis. Considering residual plots (Fig. 5), it could be found that there is a relatively linear behavior in the normal probability plot, which illustrates a normal data distribution. A random residual distribution versus fitted values suggests equal variance distributions that are vital for a favorable regression.

Fig. 5 . Residual analysis to evaluate the proposed model adequacy. (A) Normal probability plot shows a linear behavior. (B) Residual plot versus fitted value is a random distribution.

In the second step, ANOVA was done to check the model adequacy (Table 4). All linear and square terms in the model are significant with 95% of the confident level (P < 0.05). The coefficient of multiple determination R² shows relatively a good result about 92% indicating that the proposed model will probably explain a high percentage of the variability in data. Non-significant being of the lack of fit (P > 0.05) explains that there have been enough replications then the introduced model could well describe the achieved data.

Table 4. ANOVA for the proposed quadratic model
Source	DF	F	P
Blocks	2	2.79	0.114
Regression	4	23.19	0.000
Linear	2	6.65	0.017
X1	1	8.99	0.015
X2	1	10.85	0.009
Square	2	8.79	0.008
X1.X1	1	12.18	0.007
X2.X2	1	14.13	0.004
Residual error	9
Lack of fit	7	0.75	0.675
Pure error	2
Total	15
R2	92%

To compare the model estimated and experimentally produced data, the predicted values and coincidence test results were plotted simultaneously (Fig. 6) that shows a fair agreement. Western blot analysis of some samples of the CCD experiments was done to verify the periplasmic scFv expression (Fig. 7). The obtained results prove that the scFv production level is influenced by the different conditions of the CCD runs.

Fig. 6. Model estimated values against experimental obtained ones show a good agreement.

Fig. 7. Western blot analysis for samples from CCD experiments. The numbers on top of the samples are the run numbers of the experiments in the CCD.

Optimum space for the scFv production

Fig. 8A shows the contour plot of the periplasmic scFv concentrations (mg/L of media) in terms of sucrose and sorbitol molarities. It indicates acentric circles in which scFv concentrations increase by moving from the margin to the center area. The most internal circle plane is where the optimum scFv production could be reached with the 95% confidence. This region indicates a sugar made osmotic area in which the total sugar molarity (sum of sucrose and sorbitol concentrations) is around 0.8 to 0.9. This hypertonic condition was imposed on the cells during the induction period. A geometric view of the estimated model is shown in the surface plot that exhibits a maximum nature of the response surface in terms of sucrose and sorbitol concentrations (Fig. 8b). It illustrates that higher osmotic conditions imposed by the high sugar concentrations decrease the periplasmic scFv concentrations.

Fig. 8. Contour plot (A) and 3-D graph (B) of periplasmic scFv concentration in terms of sucrose and sorbitol concentrations.

Discussion

Investigating the aspects of the production process of scFvs could be a complementary to the molecular improvements to obtain the higher production yields. Therefore, considering and optimizing the physical and chemical factors affecting the production process were selected as the goal of the research.

The agitation speed and filling volume fraction are physical factors related to mass transfer and oxygen transfer rates were studied. The specified amounts of these factors, after screening effective parameters, are in good agreement with the same research in a 50 mL TubeSpin reactor system.²⁷

The induction process is one of the main steps in the recombinant proteins production process. Choosing the induction and the feeding regimes significantly could affect the productivity.^28-31 Regarding safety and economic aspects of the biopharmaceutical production, selection and utilizing a nontoxic and cheap inducers are vital for the large-scale production of the recombinant proteins.,³³ In this study arabinose was used as a suitable inducer and replaced with the IPTG as a commonly used inducer for the lab scale investigations. The arabinose is a pentose sugar that is more safe and economic than IPTG then it could be considered for the industrial scale applications. The capability of the induction by the low concentrations of the arabinose (0.1% w/w) is another satisfactory specification. The induction process is intrinsically known as a stress source that is imposed to the microorganism to express the target proteins. Hence the lower inducer concentration is used the less burden would be on the microorganism.¹³

Concentrations of the sucrose and sorbitol in the induction media are another chemical factors that by producing osmotic condition lead to higher scFv production yields. Generally, microorganisms respond to the environmental stress conditions by accumulation or lowering amount of some compounds within the cells.^15,34 Increasing the osmotic condition of the media may cause to reduce the content of the osmoregulated periplasmic glucans.³⁵ This phenomenon may be a driving force to increase the periplasmic content of the expressed scFv antibody that could be a subject for the further studies.

To evaluate quantitatively the effect of osmotic condition on the periplasmic scFv concentration, experimental designs in several steps were done. Linear normal probability plot and random residual distribution versus fitted values exhibited a normal data distribution that led to a good data regression. The derived model presented in previous section proposes a finite osmotic region in terms of the sorbitol and sucrose molarities that the optimum periplasmic scFv concentrations are obtained. Increasing further osmotic conditions impose the adverse effect on the periplasmic scFv production and raise the costs of large-scale production.

Papaneophytou et al²⁰ in similar investigation selected four effective parameters (cell density, inducer amount, temperature, and induction duration) to be optimized using the RSM. They reported the effectiveness of this method to enhance the soluble protein production from 1.8 to 9.2 mg/L.

Conclusion

Extraction and purification processes have large influences on economical aspect of the production of the commercial proteins. Expressed target protein is normally beneficial if secreted to periplasmic space or excreted to the culture media.³⁶ Identification and optimization of the key chemical and physical factors such as the culture media formulation and induction conditions could promote the expression of the periplasmic or excreted scFv and the productivity of the desired protein.³⁷

Cultures grown in the minimum media were induced in the stationary phase by the arabinose, exhibited periplasmic scFv expression. The effects of the several physical and chemical factors on the scFv production were evaluated. Regarding the results, concentrations of the sucrose and sorbitol showed significant effects on the response. Therefore they were used for further experimentations. Statistical experimental design and analysis were done based on the concentrations of these sugars producing osmotic induction media. Geometric graphs exhibited a circle region in terms of sugar concentrations in which optimum periplasmic scFv production that is more than four times of the average could be achieved.

However, more investigations need to be conducted to distinctly clarify the osmotic effects of sucrose and sorbitol on each step of induction, secretion and excretion processes. Other alternative sugars instead of sucrose and sorbitol may also have inductive effects and should be investigated.

Ethical approval

There is none to be disclosed.

Competing interests

There is none to be disclosed.

Acknowledgments

The authors would like to acknowledge Tabriz University of Medical Sciences for supporting this work.

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